The present invention relates to a magnetic sensor and more particularly to a magnetic sensor for detecting the leading edge of a ferrous target as it passes the centerline of the sensor.
Known magnetic sensors, separated from a ferrous target by a gap, typically measure the rate of change of magnetic flux in the gap as the target passes the sensor to provide a signal representative thereof. The rate of change signal is then applied directly to a threshold circuit which compares the signal to a reference to provide an output pulse when the reference is exceeded. The time of occurrence of the leading edge of the output pulse from such a sensor varies as a function of the peak level of the rate of change signal which, in turn, varies as a function of the size of the gap and the speed of the target.
For a relatively narrow target such as a tooth of a rotating gear or a narrow projection on a rotating shaft, in the known magnetic sensors, hysteresis may be added to the threshold circuit to adjust the circuit such that the trailing edge of the output pulse corresponds to the time at which the centerline of the target is coincident with the centerline of the sensor. The output pulse of the threshold circuit may then be used in applications requiring precise timing of the pulse. When the target is relatively wide, in precise timing applications, the time at which the leading edge of the target passes the centerline of the sensor becomes important. However, when known magnetic sensors are used to detect relatively wide targets, neither the leading nor trailing edge of the sensor output provides a precise indication of the time at which the leading edge of the target is coincident with the centerline of the sensor.